A handful of the most powerful tech companies are calling on President Obama and Congress to set limits on government surveillance efforts following a steady stream of revelations about data collected by the National Security Agency.

A brief letter, signed by eight companies including Apple, Google, and Microsoft, urges adoption of reforms to government surveillance efforts that are transparent, clearly defined by law, and subject to independent oversight. A version of the letter, which was also signed by Facebook, Twitter, LinkedIn, Yahoo, and AOL, appears in full-page ads in the Monday editions of the New York Times and Washington Post, among others.

"This summer's revelations highlighted the urgent need to reform government surveillance practices worldwide," the letter reads. "The balance in many countries has tipped too far in favor of the state and away from the rights of the individual -- rights that are enshrined in our Constitution."

The reforms, spelled out at reformgovernmentsurveillance.com, represent the greatest concerted effort by tech companies against mass government data collection of telephone, e-mail, and other Internet communications. The Web site includes testimonials from executives involved on the reform effort, including Google CEO Larry Page, Facebook CEO Mark Zuckerberg, and Yahoo CEO Marissa Mayer.

"People won't use technology they don't trust," wrote Brad Smith, Microsoft's general counsel. "Governments have put this trust at risk, and governments need to help restore it."

Specifically, the companies propose five principals of reform:

limiting governments' authority to collect users' data

oversight and accountability

transparency about government demands

respecting the free flow of information

avoiding conflicts among governments

The companies' level of involvement and knowledge of government surveillance activities has been under scrutiny since the release of documents this summer by former NSA contractor Edward Snowden. The issue seemed to take on greater prominence for companies in October, when the Washington Post reported that newly surfaced documents showed the NSA secretly accessed data from several tech giants by intercepting unencrypted Internet traffic in a program called Muscular.

The companies have said that their efforts to clear their reputations in the matter have been stymied by a US government gag order prohibiting them from disclosing more information about government requests they receive for customer data. In June, Google petitioned the Foreign Intelligence Surveillance Court that it has "a right under the First Amendment to publish" summary statistics about the requests. Microsoft followed suit, calling US government restrictions on what it can disclose a "content-based restriction on speech."